Retaining walls are commonly used for architectural and site development applications. The wall facing must withstand very high pressures exerted by backfill soils. Reinforcement and stabilization of the soil backfill in mechanically stabilized earth applications is commonly provided by grid-like sheet materials that are placed in horizontally extending layers in the soil fill behind the wall face to interlock with the soil and create a stable reinforced soil mass. Connection of the reinforcing material to the elements forming the wall face holds the wall elements in place and resists soil backfill pressures.
A preferred form of grid-like tie-back sheet material used to reinforce the soil behind a retaining wall structure, known as an integral geogrid, is commercially available from The Tensar Corporation of Atlanta, Ga. ("Tensar") and is made by the process disclosed in U.S. Pat. No. 4,374,798 ("the '798 patent"), the subject matter of which is incorporated herein in its entirety by reference. Integral geogrid tie-back sheet material may be uniaxially oriented according to the '798 patent to provide grid-like sheets including a plurality of elongated, parallel, molecularly oriented strands with transversely extending bars integrally connected thereto by less oriented or unoriented junctions, the strands, bars and junctions together defining a multiplicity of elongated openings. With biaxial stretching the bars may be oriented into elongated strands. While integral geogrids are preferred as reinforcing materials in the construction of retaining walls, other forms of tie-back sheet materials have been used in a similar manner.
Use of pre-cast concrete structures for wall-facing elements in a retaining wall traditionally requires an area behind the retaining wall of approximately 70% of the total wall height. This clearance area is required to accommodate the horizontally extending geogrid extending rearwardly into the fill material to anchor the wall. However, in some environments, only a limited depth of excavation is possible due to preexisting conditions, such as buried utility lines or the presence of a rock face, for example. Accordingly, alternate systems have been developed to accommodate a limited depth excavation for erection of a retaining wall. These limited depth excavations are referred to as a "cut wall" operation.
One known system for erecting a retaining wall in a cut wall environment of limited depth, is commercially available under the trade name DOUBLEWAL.RTM.. Discussion of this system is included in U.S. Pat. Nos. 4,196,161, 4,251,196, 4,351,507, 4,372,091, 4,474,400 and D-274,762, the subject matter of each of which is incorporated herein by reference for further background.
The DOUBLEWAL.RTM. system confines a significant mass of soil fill in massive concrete structural units having a pair of spaced side panels interconnected by connecting arms spaced inwardly from the ends of the side panels. By positioning adjacent structural units together, a series of approximately square shaped boxes are formed for receipt of the fill material.
A mortise-tendon connection is formed between superimposed units stacked vertically so that a vertically recessed mortise at the bottom of each superimposed connecting arm fits within a complementary shaped vertically projecting tendon at the top of each arm of a lower course of structural units. The units may be stacked vertically in alignment or in horizontally staggered rows with horizontal displacement between units and adjacent rows equal to approximately one half the length of a unit. The stacked units provide columnar openings which are filled for added structural integrity of the wall so as to form a gravity type wall.
Another system for erecting a retaining wall in a limited depth, cut wall environment is commercially available under the name T-WALL and is described in U.S. Pat. No. 4,684,294, the subject matter of which is also incorporated herein by reference for background. In this system, a concrete form includes a front face and a centrally located, rearwardly projecting stem. The stem extends into and firmly engages with a soil mass located behind the front face. Nothches in the stem provide for interengagement with support beams in the soil mass.
Granular backfill material is deposited directly on top of the stems so as to charge the compartments formed on opposite sides of the stems with equal amounts of backfill material and to prevent lateral movement of the concrete form. It is also desirable to include shear keys in a space created between teeth on the top and bottom surfaces of two superimposed stems. The purpose of these keys is to prevent movement of the unit during backfill operations.
In related application Ser. No. 08/461,850, filed Jun. 5, 1995, and assigned to Tensar, now U.S. Pat. No. 5,697,735 issued Dec. 16, 1997 the subject matter of which is incorporated herein in its entirety by reference, a system of retaining interconnected concrete panel wall elements by hoops of vertically extending sheets of grid-like material which form cells to confine fill material, is disclosed. While that system provides significant advantages in the construction of retaining walls in a cut wall environment, the large concrete wall panels themselves are cumbersome, involving complex and expensive manufacturing techniques, and requiring heavy construction equipment and skilled personnel to properly lift and place each panel. Thus, while this system is an improvement over the DOUBLEWAL.RTM. and T-WALL systems, and other systems for reinforcing concrete panel retaining walls when used in an area of limited depth, it has limited application.
In other environments, some of the disadvantages associated with the construction of concrete panel retaining walls, have been overcome with the use of modular concrete wall blocks which are relatively inexpensive to manufacture and can be stacked by a single individual, much like bricks, in superimposed, usually staggered, courses. A preferred modular wall block system for erecting a retaining wall is described in U.S. Pat. Nos. 5,540,525 and 5,595,460 assigned to Tensar, the subject of each of which is also incorporated herein by reference. In these patents, when the retaining wall is to be reinforced, horizontally extending tie-back sheets, such as the integral geogrids referred to above, are secured between selected courses of the wall blocks. To attach the geogrid, each of the wall blocks is provided with a recessed area in its upper surface, including a trough or groove adapted to receive and retain a comb-like rigid grid connection device formed of any suitable material, preferably a high strength polymer. The grid connection device includes a crossbar or spine interconnecting a multiplicity of perpendicularly extending spaced finger elements which are engaged through the openings in the end portion of the sheet of geogrid. The finger elements are forced into frictional engagement with the side wall portions of the trough and may include serrations along the edges of the finger elements to thereby securely locking the device in place. The crossbar of the connection device overlies the rearwardly extending strands of the geogrid thereby retaining the geogrid against the top surface of the block.
In the system of the '525 and '460 patents, the sheets of geogrid or the like reinforcing material generally extend horizontally over a number of modular wall blocks in a course of such blocks, and the comb-like connection devices can laterally span the gap between juxtaposed blocks. Thus, the connection of the geogrid to the retaining wall also functions to lock adjacent blocks to each other, further strengthening the integrity of the retaining wall formed therefrom.
While the modular wall block system of the '525 and '460 patents is commercially effective for many applications, it cannot be used in a cut wall environment because the depth of fill behind the blocks can seldom accommodate sufficient length of tie-back material to adequately reinforce the retaining wall, particularly if the wall is of any significant height. Therefore, there remains a need for a system adapted to reinforce a retaining wall formed of modular wall blocks in an area of limited depth.